1. Field of the Invention
The present invention relates to a power tilt cylinder device of a vessel propelling device or boat motor.
2. Discussion of the Background Art
Conventionally, as a power tilt cylinder device used for a vessel propelling device of an inboard or an outboard motor, etc., there has been available a cylinder device, which is provided between a vessel body and a propelling unit and in which the propelling unit is supported so as to be tilted against the vessel body by supplying a hydraulic fluid to or discharging a hydraulic fluid from the cylinder device. This cylinder device is composed of a cylinder, a piston rod inserted into this cylinder and extended to the outside of the cylinder via a rod guide, a piston fixed on a piston rod end part inside the cylinder for plotting and forming a first chamber of a piston rod housing side and a second chamber of a piston rod non-housing side in the cylinder and a free piston for plotting the second chamber into a piston side space and an opposite piston side space.
In the power tilt cylinder device, an upper limit position is decided by the contact of the piston with the rod guide during upward tilting, which is carried out for supplying a hydraulic fluid discharged from of a pump to the second chamber. During this period, it is necessary to control an increase in the inner pressure of the cylinder in order to protect the cylinder.
In the conventional technology for controlling an increase in the inner pressure of the cylinder during a upward tilting operation, there is available a technology, whereby in the case of a hydraulic circuit with no free pistons provided in the second chamber of the cylinder, an opening/closing valve for connecting the first and second chambers to each other and an operating member for opening this opening/closing valve are arranged in the piston, the operating member is pressed and moved by being brought into contact with the rod guide during upward tilting and operates to open the opening/closing valve of the piston, and thus the first and second chambers are caused to be communicated with each other. According to this technology, when the operating member is brought into contact with the rod guide provided in the piston in the upper limit position of upward tilting, the opening/closing valve is opened and thereby the first and second chambers are caused to be communicated with each other. As a result, the fluid, which has been supplied to the second chamber, flows away to the first chamber, and this is then discharged from the first chamber and an increase in the inner pressure of the cylinder can be controlled. However, in this conventional technology, if a free piston is provided in the second chamber of the cylinder, even when the opening/closing valve of the piston is opened, and flow of a fluid supplied to the opposite piston side space of the free piston in the second chamber is interrupted by the free piston, and this makes it impossible for a fluid to flow away to the first chamber side as that described above. For this reason, a free piston cannot be provided.
Furthermore, in the foregoing conventional technology, a shock valve is provided in the piston. When the pressure of the first chamber of the cylinder suddenly increases, which occurs, as for instance during rapid movement of a propelling unit period caused by a collision between the running propelling unit and an underwater obstacle, this shock valve serves to cause this hydraulic fluid to escape from the first chamber to the second chamber. However, a return valve for returning the hydraulic fluid from the second chamber to the first chamber after such a collision cannot be provided in the piston. This is because if such a return valve is provided in the piston, which does not have any free pistons, a hydraulic fluid supplied to the second chamber for upward tilting flows away through the return valve to the first chamber and this makes it impossible to perform upward tilting. That is, this conventional technology is disadvantaged by the fact that since a return valve cannot be provided in the piston, the propelling unit cannot return to its original position immediately after the upward movement caused by its collision with an obstacle.
Efforts were made to eliminate this disadvantage. For example, there was disclosed a device in Japanese Unexamined Patent Publication (JP-A) No. 60-1097, in which a free piston is provided in the second chamber of the cylinder and the piston is equipped with both shock and return valves. According to this device, since the piston has the return valve, the propelling unit can return to its original position immediately after the upward movement caused by its collision with an obstacle.
However, in a power tilt cylinder device like that disclosed in Japanese Unexamined Patent Publication (JP-A) No. 60-1097, as described above, it is impossible to control an increase in the inner pressure of the cylinder by providing in the piston an opening/closing valve, which is opened in the upper limit position of upward tilting for causing the first and second chambers to be communicated with each other, and causing a hydraulic fluid to escape from the second to the first chamber.
Therefore, in the conventional technology described in this Japanese Unexamined Patent Publication (JP-A) No. 60-1097, in a duct line for interconnecting a pump and the second chamber, a relief valve for escaping the inner pressure increase of the second chamber during upward tilting to a reservoir is provided. It is necessary to set the opening pressure of this relief valve to a pressure higher than that of the second chamber in order that the valve may not be opened by the pressure of the second chamber during upward tilting (pump discharging pressure) and the normal running position of the propelling unit may be stably maintained. Therefore, each time upward tilting reaches its upper limit position, the relief valve is opened after the discharging pressure of the pump increases to exceed the high opening pressure of the relief valve. As a result, improvements are required in terms of power consumption and pump durability.